One example of driver and load is an LED and its associated driver. In this description and claims, the term “LED” will be used to denote both organic and inorganic LEDs, and the invention can be applied to both categories as well as to non-lighting applications. The detailed examples below are based on OLEDs but all examples can use inorganic LEDs instead.
LEDs are current driven lighting units. They are driven using an LED driver which delivers a desired current to the LED.
The required current to be supplied varies for different lighting units, and for different configurations of lighting unit. The latest LED drivers are designed to have sufficient flexibility that they can be used for a wide range of different lighting units, and for a range of numbers of lighting units.
To enable this flexibility, it is known for the driver to operate within a so-called “operating window”. An operating window defines a relationship between the output voltage and output current than can be delivered by the driver. Providing the requirements of a particular lighting load fall within this operating window, the driver is able to be configured for use with that particular lighting load, giving the desired driver flexibility.
The driver has its output current set to the desired level within its operating window. This can be achieved by programming the driver to deliver a specific current or by providing current setting information using an input to the driver. This input can be connected to a setting resistor or other component, outside the driver, which is read by the driver. The value of the current setting resistor or other component is measured by the driver, which can then configure its output accordingly, so that the output current is determined by the resistance value. The important point is that after the driver has been produced, the output current can be selected, so that a single driver design is suitable for a range of output currents.
Once the current has been set, the voltage delivered by the driver will vary depending on the load presented to it (since the LEDs are current driven), but the driver will maintain this voltage within the operating window.
There is a particular need for a flexible driver because OLED technology is quite new and developing fast. Times between innovation of new materials and OLED architectures to give improved performance data (lumen, brightness, efficiency, size, . . . ) are very short, for example compared to typical support periods for products using the OLEDs. This support period is typically in the range of multiple years. Driver electronics also develops quickly to keep up with the demands of the new devices, particularly as driver architectures from historical LED technology cannot be simply copied to support OLEDs as well.
Although lifetime and reliability of OLEDs is also continuously improving, failed products have to be replaced. The required performance of typical devices requires the implementation of multiple OLEDs per luminaire. There is a need to be able to exchange one OLED within such a device, and to then use an updated OLED device design. For example, it is desired not to produce old device architectures longer than required, so that all production time can be allocated to state of the art devices.
One way to support older OLEDs with newer drivers or drive newer OLEDs in applications equipped also with older devices is to provide a flexible driver which knows how to drive the OLED appropriately (reduced current, dedicated dimming levels, colour point corrected . . . ), and this is enabled by the current setting resistor (or capacitor) as mentioned above. These components can be provided on a PCB attached to the OLED.
A drawback of this approach is that everything added to the back of the OLED contributes to the overall thickness of the luminaire/module.